Hexagonal-Orthorhombic Phase Transition and Ferroelasticity in K2SO4 and K2SeO4

It is shown that (011) and (031) twin planes can be introduced in the orthorhombic phase of K 2 SO 4 and K 2 SeO 4 , and both twin planes can be moved smoothly through crystals by appropriate external stress in the temperature range from about 300°C to the orthorhombic-hexagonal phase transition point. It is inferred from the crystal structure in the orthorhombic phase that SO 4 - (or SeO 4 - ) radicals must have two stable states (the up and down states), and change from one state to another during the passage of twin planes. It is suggested that the space group of the hexagonal phase is D 6h 4 –P6 3 /mmc, and the hexagonal-orthorhombic phase transition is essentially due to the ordering of SO 4 - (or SeO 4 - ) radicals.